Does Population Affect Gravity?

[brewAP2010]

Ok in my physics class we are doing planetary motion and stuff, we started talking about how the population of the Earth affects the force of gravity. So my question is does it? Also if say all of Earth's population was concentrated to one hemisphere and everyone jumped at the same time would it be possible to move the earth? Because when you jump just as you move up doesn't the Earth move down as well? haha these may seem like pretty dumb questions but i was just wondering.

 

[Delzac]

The Earth wobble. But once all the humans come back down, everything will go back being the same. Since the centre of mass essentially doesn't change. You can do a calculate if you wish, but the number will be pretty small.

 

[NobodySpecial]

It would change the length of a day though - even the weather as snow/rain accumalets changes the day winter-summer

 

[Delzac]

Hmmmmm, when you mean the length of the day, do you mean angular frequency of earth?

Since the Earth is a closed system, anything that happens inside it shouldn't change the angular momentum, and hence angular frequency isn't?

 

[NobodySpecial]

Angular momentum is conserved - but that doesn't mean angular speed (or frequency) is conserved if you can change the radius (ie moment of intertia)

 

[Delzac]

I see. I thought you were talking about the jumping part.

 

[DaveC426913]

Two things would happen:
1] As the humans jumped their 3 or so feet off the ground, the Earth would (if it were perfectly rigid, which it is not) move about an angstrom in the opposite direction. Humans and Earth would come back together, exactly as before - with no net movement i.e. taking measurements of the Earth's position and velocity before and after the event would read the same as if the event had never occurred.

2] The Earth would ring like a bell, as it does with earthquakes.

Other than that, nada.

Angular momentum is conserved - but that doesn't mean angular speed (or frequency) is conserved if you can change the radius (ie moment of intertia)

7 billion humans concentrating on one hemisphere would not change the length of day.

What would change the length of day would be if those 7 billion humans changed their latitudinal distribution; i.e. concentrated near the poles or the equator.

 

[Delzac]

What about the fact that the all the human population suddenly gather at one place? Wouldn't the moment of inertia change like what Nobodyspecial mentioned?

Edit: Woopss. You answered it.

 

[Delzac]

Since you are at it, what do you mean by latitudinal distribution?

 

[DaveC426913]

Since you are at it, what do you mean by latitudinal distribution?

Also answered. Sorry, like to edit a lot.

 

[Delzac]

Great! Thanks for the input.

 

[brewAP2010]

So basically it would move but the amount would be too small to make a difference? And then it would return to its original place or no?

 

[DaveC426913]

So basically it would move but the amount would be too small to make a difference? And then it would return to its original place or no?

Yes.

 

[brewAP2010]

ok thanks a lot.